The medial olivocochlear (MOC) branch of the auditory efferent system reduces the amplification mechanism of the outer hair cells, which may aid in signal detection in noise (Guinan, 2006). Because the MOC system is activated reflexively by sound (MOC reflex, or MOCR), its activity can be assessed by measuring otoacoustic emissions (OAEs), low-level sounds that are byproducts of outer hair cell-based amplification, and then repeating the OAE measurements with noise presented to the contralateral ear (e.g., Berlin et al., 1993). The MOCR activation by the noise reduces the amplitudes of OAEs (MOCR inhibition). Speech perception in noise performance is correlated with the magnitude of MOCR inhibition (e.g., Kumar & Vanaja, 2004), further suggesting that the MOCR is involved with hearing in noise. Previous studies have typically examined the relationship between MOCR inhibition and speech perception in noise using a single speech task. The first specific aim of the current study is to more clearly delineate this relationship by assessing speech perception in noise using multiple speech tasks (word and sentence recognition using either adaptive or fixed signal-to-noise ratios) in the same group of subjects. Both MOCR inhibition and speech perception abilities may be improved as a result of auditory training (de Boer & Thornton, 2008; Kumar et al., 2010), suggesting that MOCR inhibition may be a biomarker for auditory perceptual learning. Biomarkers may be used to identify the physiologic processes impacted by auditory training and may be predictive of the benefit obtained from training (de Boer & Thornton). These studies only examined young normal-hearing participants, so the generalizability to populations with speech-in-noise difficulties (e.g., older and hearing-impaired adults) is not known. The second specific aim of this study is to determine whether MOCR inhibition serves as a biomarker of auditory perceptual learning in a group of older adults with normal hearing or mild hearing loss. A total of 30 subjects ages 50-89 will be recruited. Subjects will be randomly assigned to an experimental or control group. The experimental group will undergo 15 hours of auditory training occurring across 10 lab visits. Auditory training will target the identification of phonemes and sentences in background noise in an adaptive manner. MOCR inhibition and speech perception in noise will be measured before, during, and after training. The control group (no training) will complete 4 visits, where MOCR inhibition and speech perception in noise will be measured to establish test-retest reliability. It is hypothesize that MOCR inhibition will be significantly correlated with all speech perception measures, with the largest correlations occurring for the more challenging signal-to-noise ratio conditions. It is also hypothesized that the change in MOCR inhibition across time in the experimental group will be significantly correlated with the changes seen in speech perception. The results of this study will have implications for understanding the physiologic mechanisms involved in speech-in-noise perception and for objectively assessing the benefit from auditory training.